The Perhentian Islands get affected by the monsoon season, which lasts from November to January/February. Only in December do boats cease traveling between Kuala Besut Jetty and the islands.
The high season is between March and September, when accommodation prices are much higher than during the low season, which is between October and early February. The low season is probably the best time to visit the Perhentian Islands, as there are fewer tourists.
Mosquitoes are quite rampant and rather ferocious on Perhentian Besar and Kecil, so don't leave your insect repellent cream at home.